Force feedback is used primarily with position control systems wherein the position of a master is mapped to correspond with the position of the slave. In these systems the forces adjacent to an endpoint on the arm being controlled are determined or sensed and corresponding scaled forces are applied to the master or hand control so that the operator is subjected to an indication of the forces on the endpoint based on the scaled forces which the operator must overcome or otherwise accommodate in moving the master to obtain the desired movement of the slave.
The concept of force feedback in conjunction with a velocity control system wherein the velocity of an end point is controlled in any direction by the degree of displacement of a manual controller (master) in a corresponding direction has been suggested, but has not proved successful.
U.S. Pat. No. 5,116,180 issued May 26, 1992 to Fung et al. discloses both a position control system and a velocity control system each of which when operated in the active mode applies force feedback to the master which reflect or represent in some manner the forces applied at the end point.
When this system is operated in the active mode as a velocity or rate controller the scaled forces applied to the hand controller are a function solely of the actual force applied at the end point and as a result the operation is unstable, produces an unnatural feel to the operator and is difficult to use. The velocity controller master (joystick) is biased to its datum or neutral position (d.sub.m =0), wherein no velocity signals are generated, by a spring force the magnitude of which changes solely with degree of displacement from the datum. However, the same force feedback force (scaled from the end point force) is applied to the master for a given end point force regardless of the displacement of the master, so that the portion of the force felt by the operator generated by the forces at the endpoint for a small displacement of the hand controller will be significantly larger than for a larger displacement and thus the components of the force felt by the operator change significantly with displacement of the master. As a result, the sensations received by the operator are not clearly representative of the environment of the endpoint (i.e. forces on the endpoint) being controlled and thus may be are confusing to the operator. Furthermore the force applied to the master as the datum is reached are likely to result instability in that the force applied may well tend cause the control to overshoot the datum, which in turn requires the application of large damping forces be applied to the master and these damping forces further mask the feedback forces.